1. Field of the Invention
The present invention relates to a power supply device which is mounted in a portable device such as a camera, a notebook-type personal computer, for example, to supply electric power to a motor, a microcomputer and the like provided in the portable device.
2. Description of the Related Art
Conventionally, this kind of portable device is constructed in such a manner that a battery can be used as a power supply source. For this power supply source a primary battery which cannot be charged or a secondary battery which can be charged, can be used. Output voltage of the battery drops, in accordance with the remaining capacity of the battery. Accordingly, for applying a predetermined voltage to a circuit such as a motor, the device is constructed in such a manner that the output voltage of the battery is controlled by a switching regulator. On the other hand, when the battery is used for a circuit such as a microcomputer, which requires an extremely stable voltage, the output voltage of the battery may be controlled by a series regulator.
The switching regulator has a high efficiency when the input-output voltage difference of the switching regulator is large. However, since electric current is consumed by the circuitry of the switching regulator, when the voltage of the battery becomes low, electric power consumed by the device in which the switching regulator is mounted is increased. Thus, the switching regulator has a drawback in that the operating life of the battery is shortened and noise appears in the output voltage of the switching regulator. On the other hand, in the series regulator, although noise is completely removed from the output voltage thereof, there is a drawback in that the efficiency is low when the output voltage of the battery is high, since heat is radiated in accordance with the input-output voltage difference of the series regulator.